In AM radio broadcasting in the medium-wave and short-wave bands, a high-power vacuum tube is conventionally used in the final radio frequency amplifier stage of the transmitter. For maximum power-amplification efficiency, this tube is not operated as a linear amplifier, but rather as a class C or class D biased circuit, producing an RF envelope which follows that of the B+DC supply voltage provided to the tube anode. Thus, modulation of the RF signal is achieved through varying the B+DC supply to the plate anode of the tube. The high-powered audio amplification circuitry required to vary this voltage is referred to in the art as the modulator.
Recently, a modulator to achieve the foregoing has been employed in the art and is known as a pulse step modulator (PSM). Such a pulse step modulator is disclosed in U.S. Pat. No. 4,403,197 to H. I. Swanson. Briefly, a pulse step modulator (PSM) as disclosed in that patent includes a plurality of series connected unit step modules each of which includes an isolated DC voltage source, a remotely controlled switch and a series diode. The switch in each module may be remotely controlled to turn the module on or off. As each module is turned on, it provides a step voltage. As the various modules are turned on in a stepwise fashion, the output voltage will increase in steps from 0 volts to a maximum voltage with the maximum equalling the sum of all of the module DC voltage sources. A lowpass filter at the output may be employed for removing switching noise. An encoder or the like monitors a time varying input signal, such as an audio signal, and turns on one of the unit step modules for each incremental increase in the value of the audio signal. As the audio signal continues to increase in value, the modules are turned on one at a time in a given order. Similarly, as the audio signal decreases in value, the modules are sequentially turned off in the reverse order.
The pulse step modulator described above also includes circuitry to obtain fine resolution of the output voltage by including a plurality of fractional voltage step modules in the modulator series circuit. This permits generation of output voltages that would otherwise be difficult to obtain. Consequently, the modulator circuit includes a plurality of series connected unit step modules as described above which are connected in series with a plurality of series connected binary step modules which are connected in series with each other. Each binary step module also includes an isolated DC voltage source, a remotely controlled switch and a series diode. However, the DC voltage sources are of different values from each other and from that employed in the unit step modules. Thus, each voltage source in the unit step modules has a value V. The binary modules have voltage sources that vary in value from the most significant binary module to the least significant binary module. Thus, the most significant binary module has a voltage source that is equal to one-half that of the voltage source in each unit step module or V/2. Each successive binary module has a voltage source that is of a value equal to one-half that of the previous module. Consequently, activation of the binary modules in a binary-counting sequence will create a linear increase in the output voltage.
The switches utilized by the unit step modules must be capable of handling both high voltage and current and are typically devices of the IGBT or GTO transistor families. The binary step modules, in contrast, operate at lower DC voltages so that they may utilize MOSFET transistors as switches. The switching efficiency of MOSFET transistors is considerably higher than that of either IGBT or GTO transistors and it becomes essential to the efficiency of a modulator to limit the rate of switching of the unit step modules. A high rate of switching of the switches used in the binary step modules or the unit step modules is hereinafter referred to as module dither. It is permissible to allow the binary step modules which utilize MOSFET switching devices to dither. There will exist conditions of output voltage at which one of the unit step modules may tend to dither. Due to the less efficient switching performance of the IGBT or GTO transistor switches employed in the unit step modules, if the system is allowed to operate continuously with unit step dithering, the transistor switch employed in the unit step module being dithered will likely overheat and fail.